hygrometer (hĪgrŏmˈətər) [key], instrument used to measure the moisture content of a gas, as in determining the relative humidity of air. The temperature at which dew or frost forms is a measure of the absolute humidity—the weight of water vapor per unit volume of air or other gas at the temperature before cooling. Knowing absolute humidity and air temperature, the observer can calculate relative humidity. The most common type of hygrometer is the dry- and wet-bulb psychrometer. It consists of two identical mercury or electrical thermometers, one of which has a wet cotton or linen wick around its bulb. Evaporating water from the wick absorbs heat from the thermometer bulb, causing the thermometer reading to drop. The difference between dry-bulb and wet-bulb temperatures are compared on psychrometric charts. The temperature at which dew forms, called the dew point, is determined by a dew-point, or condensation-type, hygrometer, which is basically a mirror, usually of polished metal, cooled until dew or frost forms on it. Various cooling methods employ compressed carbon dioxide, dry ice, liquid air, or mechanical refrigeration. When dew or frost forms on the mirror, the temperatures between when dew appeared and when it disappeared is the dew point and is read with optical, electrical, or nuclear techniques. Because human observation varies, photoelectric cells are sometimes used to register the time at which the mirror fogs. Another type of hygrometer works on the principle that electrical resistance varies in a material that absorbs moisture, with the resistance to a current passing between wires measured by special sensors. The most accurate way to measure humidity is with an electric hygrometer, in which a known volume of gas passes over a hygroscopic, or moisture-absorbing, material such as phosphorus pentoxide. It is weighed before and after to determine how much water it took out of the gas. The older types of mechanical hygrometer use human hair, which stretches as it absorbs moisture, with a linkage connecting the center of a bundle of hairs under tension to a pointer. Another uses goldbeater's skin, a membrane from the intestines of oxen. Some materials can determine relative changes in humidity because they change color as they absorb moisture.